1. Field of the Invention
The invention relates to a material for deaerating coating systems which are applied from an aqueous phase onto surfaces and which contain polymeric organic film formers.
2. Description of the Prior Art
On applying water-dilutable coating systems onto a substrate and during or after the evaporation of the water from the aqueous dispersion, very small gas bubbles, finely dispersed in the coherent coating film being formed, frequently develop as the film is drying. These bubbles are sometimes referred to in the art as a microfoam. A portion of these bubbles float to the surface of the film where they burst and cause no defects, provided that the film is still sufficiently flowable for levelling the surface defects. Another portion of the bubbles rises to the surface. However, these bubbles do not burst. Rather, as the vehicle film cures, these bubbles form a very thin surface skin which can easily be damaged mechanically. Other bubbles remain dispersed in the film. Such defects in a paint film are described as "pin holes".
The phenomenon of this so-called microfoam is not to be compared with the behavior and appearance of a conventional, more or less finely dispersed, polyhedral foam. Known defoamers or antifoaming agents destroy the lamellae of such a foam, that is, the walls separating the individual foam bubbles, or prevent the formation of stable foam bubbles. In a microfoam, the individual, mostly spherical gas bubbles in water-dilutable coating systems are generally separated so far apart from each other, that no lamellae are formed between individual foam spheres. For this reason, known antifoaming agents generally fail to eliminate and remove the microfoam. The process of eliminating such microfoams is often referred to as deaerating. Further references to the differences in the behavior of spherical foam and polyhedral foam are given in "Ullmanns Encyclopadie der technischen Chemie" (Ullmann's Encyclopedia of Chemical Engineering), Vol. 20, pages 441 ff.
It is known that air bubbles, enclosed in hydraulic oils, lubricating oils or pickling fluids may be removed by additives. Such additives are described in German Auslegeschrift 23 05 257. These additives, however, fail to remove the microfoam from aqueous preparations.
It is presumed that processes at the gas/liquid interface affect the deaeration. The deaeration is possibly affected by changes in the viscosity of the coating system at the interface with the gas bubbles. In any case, because of the different physical and/or chemical influences on the interfaces between liquid and gas, those skilled in the art cannot draw conclusions from the effectiveness of antifoaming agents which might be applicable to the effectiveness of deaerating agents.